1. Field of the Invention
The present invention relates to the vaporization of liquid hydrocarbon fuel for use in conventional steam reformers, thermal steam reformers, and advanced reformer concepts.
2. Description of the Prior Art
It is known to use hydrogen as a source of energy in fuel cells. One method of obtaining hydrogen fuel is by conventional steam reforming in which vaporized hydrocarbon fuel is reacted with steam in the presence of a catalyst to produce hydrogen (hydrocarbon+H.sub.2 O+energy.fwdarw.3H.sub.2 +CO.sub.2). The liquid hydrocarbon fuel which is to be reacted must be introduced into the steam reforming reactor in vapor phase.
One method of vaporizing the liquid hydrocarbon fuel is to pass the liquid through a heat exchanger to vaporize the liquid before introducing it into the steam reformer. A disadvantage with this method is that hydrocarbon residue tends to bake and accumulate on heat exchanger walls, thus reducing heat exchanger efficiency and eventually clogging the heat exchanger.
One method of reducing hydrocarbon accumulation and clogging is to mix the liquid hydrocarbon fuel with a vapor prior to introduction into the steam reforming reactor. U.S. Pat. No. 3,698,957 discloses a method wherein steam from a heat exchanger is mixed with liquid hydrocarbon fuel to produce vapor which is injected into the reformer reactor. There are two drawbacks to this method. In order for the heat exchanger to heat water into steam having a sufficiently high energy content to vaporize the liquid hydrocarbon fuel, a high temperature energy source must be used in the heat exchanger. Also the liquid hydrocarbon fuel may contain significant amounts of sulfur, which may have to be removed from the fuel before it is introduced into the reformer reactor. A conventional desulfurizer is used for this purpose. However, the desulfurizer cannot tolerate significant amounts of oxygen, and therefore, steam cannot be used to vaporize the liquid fuel.
It is also known to use recycled hydrogen vapor from the reformer reactor to vaporize the liquid hydrocarbon fuel. The hydrogen vapor must be heated in a heat exchanger prior to mixing with the liquid hydrocarbon fuel. A disadvantage of this method is that the energy source used to heat the hydrogen vapor in the heat exchanger must have a relatively high temperature.
To summarize, the prior art method of vaporizing liquid hydrocarbon fuel is a one step process wherein the vapor to be mixed with the liquid hydrocarbon is heated in heat exchangers to temperatures in excess of 1000.degree. F. In order to heat the vapor to such high temperatures, the heat exchanger must use a high temperature energy source, generally, vapor having a specific heat in the range from 0.2 to 1 Btu/lb.degree.F. and having a temperature between about 1100.degree. and 1200.degree. F.
It is an object of the present invention to provide vaporized liquid hydrocarbon fuel without necessitating reliance on high temperature energy sources for the heat exchanger.
It is a further object of the present invention to provide a method of vaporization of liquid hydrocarbon fuels that reduces hydrocarbon accumulation on heat exchangers.